1. Field of the Invention
The present invention relates to a viewing-direction changing optical system for long and slender image-transmission systems such as endoscopes.
2. Description of the Prior Art
When observing the inside of body cavity by an endoscope in which an optical fiber bundle is used, it is possible to observe in various directions in the body cavity by bending the distal end of endoscope. However, in such observing method to bend the distal end of endoscope, the observing directions are limited and there occur some directions in which observation is impossible. Moreover, when it is impossible to bend the endoscope itself as in case of an inflexible endoscope, it is possible to observe in one direction only. Therefore, in order to enable to observe in a wide range, a viewing-direction changing optical system is required. As viewing-direction changing optical systems, such optical systems as shown in FIGS. 1 through 3 are known. The optical system shown in FIGS. 1A and 1B is arranged as follows. That is, a roof prism 2 is arranged in front of objective 1 of endoscope. The observing direction is changed from the forward viewing direction to the side viewing direction by rotating the roof prism 2 from the position shown in FIG. 1A to the position shown in FIG. 1B. The optical system shown in FIG. 2 is arranged as follows. That is, two right-angled prisms 3 and 4 are arranged in front of objective 1. One prism 3 is fixed while the other prism 4 is rotated round the axis 4a (when a concave lens is arranged in front of prism 4 as shown in FIG. 2, the prism 4 is rotated integrally with the concave lens). Thus, it is possible to observe in a wide range of viewing directions. The optical system shown in FIG. 3 is arranged as follows. That is, two prisms 5 and 6 are arranged in front of objective 1. Out of those prisms, the prism 6 is rotated round the axis 6a. Thus, it is possible to observe in various directions.
Known viewing-direction changing optical systems explained in the above are all arranged to change the viewing direction by rotating the prism or one of prisms arranged in front of objective and, therefore, they need a large space and complicated mechanism for rotation of prism. Consequently, the distal end becomes large and this is not desirable for endoscopes.